Zeolite L

ABSTRACT

Zeolite L with flat basal planes, and reduced crystallite size is prepared in a synthesis modified by the addition of small amounts of additional metal such as magnesium, calcium, barium, cobalt, zinc, chromium, manganese or nickel. The addition of these metals also suppresses unwanted zeolite W formation even when the synthesis would otherwise form this zeolite.

This application is a continuation of U.S. Ser. No. 07/715,011, filedJun. 13, 1991, now abandoned, which is a continuation of U.S. Ser. No.07/501,406, filed Mar. 29, 1990, now abandoned, which is a continuationof U.S. Ser. No. 07/481,484, filed Feb. 16, 1990, now abandoned, whichis a continuation of U.S. Ser. No. 07/298,336, filed Jan. 17, 1989, nowabandoned, which is a continuation of U.S. Ser. No. 06/918,457, filedOct. 14, 1986, now abandoned.

This invention relates to a highly crystalline zeolite L, itspreparation and use in catalysts, particularly for aromatization. Inparticular, it relates to zeolite L with cylindrical morphology, whichprovides a catalyst base giving extended lifetimes in thedehydrocyclization of alkanes.

Zeolite L has been known for some time as an adsorbent, and in U.S. Pat.No. 3,216,789 is described as an aluminosilicate of the formula:

    0.9-1.3M.sub.2/n O:Al.sub.2 O.sub.3 :5.2-6.9SiO.sub.2 :yH.sub.2 O

(where M is an exchangeable cation of valence n and y is from 0 to 9)having an x-ray diffraction pattern with the following more significantd(Å) values:

    ______________________________________                                        16.1 ± 0.3                                                                 7.52 ± 0.04                                                                6.00 ± 0.04                                                                4.57 ± 0.04                                                                4.35 ± 0.04                                                                3.91 ± 0.02                                                                3.47 ± 0.02                                                                3.28 ± 0.02                                                                3.17 ± 0.01                                                                3.07 ± 0.01                                                                2.91 ± 0.01                                                                2.65 ± 0.01                                                                2.46 ± 0.01                                                                2.42 ± 0.01                                                                2.19 ± 0.01                                                                ______________________________________                                    

The preparation of zeolite L described in U.S. Pat. No. 3,216,789comprises crystallizing the zeolite from a reaction mixture comprisingmole ratios:

    ______________________________________                                        K.sub.2 O/(K.sub.2 O + Na.sub.2 O)                                                              0.33-1                                                      (K.sub.2 O + Na.sub.2 O)/SiO.sub.2                                                              0.35-0.5                                                    SiO.sub.2 /Al.sub.2 O.sub.3                                                                     10-28                                                       H.sub.2 O/(K.sub.2 O + Na.sub.2 O)                                                              15-41                                                       ______________________________________                                    

The silica to alumina ratio in this reaction mixture is significantlyhigher than the ratio in the formed zeolite.

British Patent 1,202,511 describes a revised zeolite L preparation usinglower proportions of silica in the reaction mixture which comprises moleratio of reactants as:

    ______________________________________                                        K.sub.2 O/(K.sub.2 O + Na.sub.2 O)                                                              0.7-1                                                       (K.sub.2 O + Na.sub.2 O)/SiO.sub.2                                                              0.23-0.35                                                   SiO.sub.2 /Al.sub.2 O.sub.3                                                                     6.7-9.5                                                     H.sub.2 O/(K.sub.2 O + Na.sub.2 O)                                                              10.5-50                                                     ______________________________________                                    

The ratio H₂ O/(K₂ O+Na₂ O+SiO₂ +Al₂ O₃) is preferably not greater than6 to give a "dry gel".

U.S. Pat. No. 3,867,512 discloses a preparation of zeolite L from areaction mixture having a molar composition:

    ______________________________________                                        K.sub.2 O/(K.sub.2 O + Na.sub.2 O)                                                              0.3-1                                                       (K.sub.2 O + Na.sub.2 O)/SiO.sub.2                                                              0.3-0.6                                                     SiO.sub.2 /Al.sub.2 O.sub.3                                                                     10-40                                                       H.sub.2 O/(K.sub.2 O + Na.sub.2 O)                                                               45-140                                                     ______________________________________                                    

in which the silica source is a gel having at least 4.5 weight percentwater and prepared in a particular manner.

L. Wilkosz in Pr Chem 409 (1974)--Chemical Abstracts, Vol. 90 (1979)573478 describes the preparation of zeolite L from a synthesis solprepared by treating a solution containing silica, potassium hydroxideand sodium hydroxide with a second solution containing potassiumaluminate, potassium hydroxide and sodium hydroxide and crystallizingfor 72 hours at 20° C. and 122 hours at 100° C. The zeolite L producthas a SiO₂ :Al₂ O₃ ratio of 6.4:1.

G. V. Tsitsishvilli et al in Doklady Akademii NaukSSSR, Vol. 243, No. 2,pp 438-440 (1978) describe the synthesis of zeolite L fromalumina-silica gels containing tributylamine. The gels used had thefollowing molar ratios:

    ______________________________________                                        SiO.sub.2 :Al.sub.2 O.sub.3                                                                      25                                                         (K.sub.2 O + Na.sub.2 O):Al.sub.2 O.sub.3                                                        18                                                         (K.sub.2 O + Na.sub.2 O):SiO.sub.2                                                               0.72                                                       H.sub.2 O/K.sub.2 O + Na.sub.2 O                                                                 20                                                         K.sub.2 O:Na.sub.2 O                                                                             0.5                                                        ______________________________________                                    

Y. Nishiimura in Nippon Kagaku Zasshi 91, 11, 1970, pp 1046-9 describesin general terms zeolite L preparation from a synthesis mixturecontaining colloidal silica, potassium aluminate and potassium hydroxidehaving a SiO₂ :Al₂ O₃ ratio of 15-25, but exemplifies only two synthesismixtures having the following ratios of components:

    7K.sub.2 O:Al.sub.2 O.sub.3 :20SiO.sub.2 :450H.sub.2 O

and

    8K.sub.2 O:Al.sub.2 O.sub.3 :10SiO.sub.2 :500H.sub.2 O

Frety et al in C.R. Acad Sc. Paris, t275, Serie c-1215 describes theelectron microscopy examination of zeolite L in which particles weresaid to be observed in the form of slightly deformed cylinders with veryvariable dimensions.

U.S. Pat. No. 3,298,780 describes a related zeolite known as UJ preparedfrom an aqueous reactant solution having a composition, expressed asmolar ratios of oxides, corresponding to:

    SiO.sub.2 /Al.sub.2 O.sub.3 of from 6 to 30

    R.sub.2/u O/SiO.sub.2 of from 0.30 to 0.70,

and

    H.sub.2 O/R.sub.2/u O of from 80 to 140

at a temperature between 150° F. (65.6° C.) and 325° F. (162.8° C.)until the zeolite crystals are formed.

Zeolite UJ is described as having nearly cubic shaped crystals with acrystal size ranging upward from 0.05 micron.

GB 1,393,365 describes a further related zeolite known as AG1 which isprepared by reacting at least one aluminium component, at least onesilicon component and at least one alkali metal component, in an aqueousmedium, the sole or major silicon component being a water glass having amolar ratio SiO₂ /M₂ O of 3.5 to 4.0 to give a reaction mixture withoxide molar ratios in one of the following ranges:

    ______________________________________                                        Range 1                                                                       SiO.sub.2 /Al.sub.2 O.sub.3                                                                      7-14                                                       (K.sub.2 O + NaO.sub.2)/SiO.sub.2                                                               0.25-0.85                                                   K.sub.2 O/(K.sub.2 O + Na.sub.2 O)                                                              0.75-1                                                      H.sub.2 O/(K.sub.2 O + Na.sub.2 O)                                                              25-160                                                      Range 2                                                                       SiO.sub.2 /Al.sub.2 O.sub.3                                                                     14-20                                                       (K.sub.2 O + Na.sub.2 O)/SiO.sub.2                                                              0.25-0.85                                                   K.sub.2 O/(K.sub.2 O + Na.sub.2 O)                                                              0.5-1                                                       H.sub.2 O/(K.sub.2 O + Na.sub.2 O)                                                               25-160                                                     Range 3                                                                       SiO.sub.2 /Al.sub.2 O.sub.3                                                                     20-40                                                       (K.sub.2 O + Na.sub.2 O)/SiO.sub.2                                                              0.25-1                                                      K.sub.2 O/(K.sub.2 O + Na.sub.2 O)                                                              0.4-1                                                       H.sub.2 O/(K.sub.2 O + Na.sub.2 O)                                                               25-160                                                     ______________________________________                                    

EP0096479 describes zeolite L crystallites in the form of distinctcircular cylinders with a mean diameter of at least 0.5 micron and anaspect ratio of at least 0.5, and their preparation in which an alkalinereaction mixture comprising water, a source of silicon and a source ofaluminium with a composition falling within the following molar ratios(expressed as oxides):

    ______________________________________                                               M.sub.2/n O/SiO.sub.2                                                                  0.22-0.36                                                            H.sub.2 O/M.sub.2 O                                                                    25-90                                                                SiO.sub.2 /Al.sub.2 O.sub.3                                                             6-15                                                         ______________________________________                                    

(wherein M is a cation of valence n, and preferably potassium or amixture of K+M¹ in which M¹ is an alkali metal or alkaline earth metalsuch as sodium, calcium, barium, or rubidium, provided that K_(2/n) O/M¹₂ O+K₂ O) is at least 0.7) is heated to a temperature of from at least75° C. and preferably from 100° C. to 250° C., more preferably from 120°C. to 225° C., to form the desired cylindrical aluminosilicate.

EP0142353 describes a process for the preparation of zeolite L in whichan alkaline reaction mixture comprising water, a source of silicon and asource of aluminium with a composition having the following molar ratio(expressed as oxides):

    ______________________________________                                               M.sub.2/n O/SiO.sub.2                                                                 0.22-0.30                                                             H.sub.2 O/M.sub.2 O                                                                   25-45                                                                 SiO.sub.2 /Al.sub.2 O.sub.3                                                           10-12                                                          ______________________________________                                    

(wherein M is as defined hereinbefore) is heated to a temperature ofgreater than 150° C. for a period long enough to form zeolite L.

EP0142354 describes a process for the preparation of zeolite L in whichan alkaline reaction mixture comprising water, a source of silicon and asource of aluminium with a composition having the following molar 5ratio (expressed in oxides):

    ______________________________________                                               M.sub.2/n O/SiO.sub.2                                                                 0.24-0.35                                                             H.sub.2 O/M.sub.2 O                                                                   35-80                                                                 SiO.sub.2 /Al.sub.2 O.sub.3                                                           5.7-7.8                                                        ______________________________________                                    

(wherein M is as defined hereinbefore) is heated to a temperature ofgreater than 200° C. to form zeolite L.

Further zeolite L preparations are described in EP0142355, EP0142347,EP0142348 and EP0142349.

It was subsequently found that zeolite L may be used as a catalyst basein aromatization reactions. U.S. Pat. No. 4,104,320 discloseddehydrocyclization of aliphatic compounds in the presence of hydrogenusing a catalyst comprising zeolite L and a group VIII metal, in whichthe zeolite L is of the formula:

    M.sub.9 /n(AlO.sub.2).sub.9 (SiO.sub.2).sub.27

(where M is a cation of valence n) but the silica to alumina ratio mayvary from 5 to 7. Zeolite L is described as occuring in the form ofcylindrical crystals a few hundred Angstroms in diameter.

East German Patent 88789 discloses dehydrocyclization using a catalystformed from a zeolite precursor with a silica to alumina ratio of up to70. Zeolite L is mentioned as a precursor.

European Patent Application Publication 40119 discloses adehydrocyclization process operating at low pressure (1 to 7 bars) orlow H₂ /hydrocarbon ratio using a catalyst comprising platinum on apotassium zeolite L. BE 888365 describes dehydrocyclization using acatalyst comprising platinum, rhenium (incorporated in the form of itscarbonyl) and sulphur to give an atomic ratio of sulphur to platinum of0.05 to 0.6 on a zeolitic crystalline aluminosilicate base such aszeolite L. BE792608 discloses the treatment of zeolite L for use ascatalyst in isomerization by exchange with ammonium and chromium ions.EP0142351 describes a method for improving the dispersion of noble metalparticles on zeolite L to provide an improved reforming catalyst.EP0145289 describes a catalyst comprising zeolite L and highly dispersedparticles of a Group VIII metal.

GB2116450 describes a zeolite catalyst comprising a zeolite of the Lfamily, at least one Group VIII metal and an alkaline earth metalselected from barium, strontium and calcium. The catalyst is used forreforming, dehydrocyclizing acyclic hydrocarbons, dehydroisomerizingalkylcyclopentanes and dealkylating toluene. Process using such catalystare described in GB2114150 and GB2142648.

It has now been found that an improved zeolite L, but having acharacteristic morphology and/or size and/or cation content and/orsilica/alumina ratio is particularly valuable for use as a catalyst basein hydrocarbon conversions such as aromatization.

The prior art, and particularly EP 0096479, EP 0142353 and EP 0142354,teaches that in zeolite L synthesis a reduction in alkalinity leads tozeolite W formation, and also to larger crystallites. It hassurprisingly now been found that by incorporation of certain additionalmetals in the synthesis gel zeolite L may be made at low alkalinitiesand under conditions which normally favour formation of other zeolitespecies. Furthermore small crystallites may be obtained with aparticular shape which has shown advantages in catalyst performance.

Thus, in one aspect this invention concerns zeolite L comprisingcrystallites in the form of cylinders with basal planes of such a shapethat the ratio of axial length of curved cylindrical surface (m) to theoverall axial length of the crystallite (h) is greater than 0.9, andpreferably approaches 1. A geometrically perfect cylinder withcompletely flat basal planes would have m=h, and m/h=1, while any domingor growths on the basal surfaces mean that h is greater than m, and m/his less than 1. We have now found that more perfect cylinders, withflatter basal planes than obtained in EP0096479, result in a zeolite Lproduct with better catalytic performance.

The cylindrical crystallites preferably have a mean diameter (d) of atleast 0.05μ, more preferably at least 0.1μ. The aspect ratio (the ratioof the axial length of the cylindrical surface m to the mean diameter d)is preferably at least 0.5, more preferably at least 0.75 and mostpreferably at least 1.

A particularly preferred zeolite L of the invention comprisescrystallites in the form of well-defined, smooth-surfaced cylinders withsubstantially flat basal planes, and thus a m/h ratio of substantiallyunity, a mean diameter d of from 0.1 to 0.5μ, and an aspect ratio of(m/d) from 0.75 to 5.

Preferably the zeolite L comprises cylindrical crystallites wherein atleast 80%, more preferably at least 90%, of the basal planes aremicroscopically flat to within 200 Å, and thus do not exhibit spiralstep growths thereon.

The zeolite L of the invention may also have a relatively highsilica/alumina ratio. The cylindrical crystallites described inEP0096479 have silica/alumina ratio of not greater than 6.3, and zeoliteL products described therein with higher silica/alumina ratio were inthe form of clams or discs (that is, with an aspect ratio of less than0.5). Thus, a preferred aspect of the invention comprises zeolite L inthe form of cylindrical crystallites having an aspect ratio of at least0.5 and a silica/alumina ratio of greater than 6.5.

The zeolite L of the invention is characterised by its cylindricalmorphology. The terms "cylinder" and "cylindrical" are used herein todescribe the shape of a cylinder as defined in solid geometry--that is,a solid bounded by a surface generated by a line moving parallel to afixed line so as to cut a fixed plane curve and by two parallel planes(bases) which cut the surface. The cylinders will generally be circularcylinders, that is, with circular cross-section, but in the context ofthe invention the cylinders may also exhibit some flattening of thecylindrical surface such that the cross-section has polygonal, andparticularly hexagonal character--that is to say, is in the form of acurvilinear hexagon--and the terms "cylinder" and "cylindrical" are usedto include such forms.

The zeolite L of the invention displays an x-ray diffraction patterntypical for zeolite L, subject to the changes in position and intensityof the x-ray lines discussed in EP0096479. Occasionally, additionallines not belonging to the pattern for zeolite L appear in a patternalong with the x-ray lines characteristic of the zeolite. This is anindication that one or more additional crystalline materials are mixedwith zeolite L in the sample being tested. It is a preferred feature ofthe invention that the amount of such additional crystalline materialsis minimised in the zeolite material as synthesized. In particular, itis preferred that the synthesis of the zeolite of the invention isconducted so that the amount of zeolite W in the product of thesynthesis is minimised. Further, the synthesis of the zeolite of theinvention is preferably conducted such that the product of the synthesisis substantially free of any additional crystalline phase giving rise toa line in the x-ray pattern at d (Å) value of 6.28±0.05.

The zeolites of the invention are preferably aluminosilicates and willbe described hereinafter in terms of aluminosilicates, though otherelemental substitutions are possible, for example aluminium may besubstituted by gallium, boron, iron and similar trivalent elements, andsilicon may be substituted by elements such as germanium or phosphorus.The aluminosilicates preferably have a composition (expressed in termsof molar ratios of the constituent oxides in anhydrous form) of:

    (0.9-1.3)M.sub.2/n O:Al.sub.2 O.sub.3 :xSiO.sub.2

wherein M represents one or more cations of valence n, x is from 5 to7.5, preferably from 6.5 to 7.5. The zeolite materials of the inventionhave high crystallinity as shown by a well-defined x-ray diffractionpattern (without binder or other diluents present) with sharp peaks.

The exchangeable cation M in general formula I is potassium, but it ispossible for a part of M to be replaced by other cations such as alkalimetals for example sodium. As described hereinafter, such cations may beintroduced during synthesis. The ratio M_(2/n) O:Al₂ O₃ is preferablyfrom 0.95 to 1.15 and generally above 1.

The aluminosilicate forms of the invention may be hydrated, typicallywith from 0 to 9 moles of water per mole of Al₂ O₃. When used as acatalyst base, as described hereinafter, the zeolite of the invention ispreferably first calcined to remove water. In normal preparation fromaqueous gels a hydrated form is first prepared and this may bedehydrated by heating.

Cylindrical particles have been shown in EP0096479 to have excellentproperties of extending catalyst life when used as catlyst bases foraromatization catalysts as compared to the morphologies produced byprior art processes. It is now found that particles of this inventionprovide a means of extending catalyst life still further and/or givingsurprising increases in activity in aromatization and/or in selectivityto aromatic products and/or show increased stability.

It is further surprising feature of the invention that the improvedzeolite L with cylindrical morphology may be prepared in the presence ofsmall amounts of additional metals, preferably alkaline earth metals andcertain transition metals by controlling the composition of the reactionmixture within certain limits, which in the absence of additional metaldo not necessarily result in the desired zeolite L.

In another aspect, therefore, the invention provides a process for thepreparation of zeolite L crystallites in the form of cylinders, in whichan alkaline reaction mixture comprising water, a source of an alkalimetal, a source of silicon, a source of aluminium and a source of anadditional metal M¹¹, with a composition falling within the followingmolar ratios (expressed as oxides):

    ______________________________________                                        (M.sup.1.sub.2 O + M.sup.11.sub.2/n O)/SiO.sub.2                                                0.18-0.36                                                   H.sub.2 O/(M.sup.1.sub.2 O + M.sup.11.sub.2/n O)                                                25-90                                                       SiO.sub.2 /Al.sub.2 O.sub.3                                                                      5-15                                                       M.sup.1.sub.2 O/M.sup.1.sub.2 O + M.sup.11.sub.2/n O)                                             0.9-0.9999                                                ______________________________________                                    

(wherein M¹ is an alkali metal, M¹¹ is magnesium, calcium, barium,manganese, chromium, cobalt, nickel or zinc cation and n is the valenceof M¹¹) is heated to a temperature of from at least 75° C. andpreferably from 100° C. to 250° C., more preferably from 120° C. to 225°C., to form the zeolite L of the invention.

There are five principal components to the reaction mixture of synthesisgel and thus generally:

aluminium

silicon

alkali metal, preferably potassium

additional metal M¹¹

water

and the relative proportions of these components and the chosen reactionconditions are important if the desired zeolite L of the invention is tobe obtained.

Zeolite W tends to be formed as a contaminant in zeolite L preparationat some extremes of gel composition. It is advantageous for the zeoliteW content of the product to be minimized. The zeolite W content of theproduct can be monitored by its x-ray diffraction pattern.

A characteristic prominent line 35 in the zeolite W XRD pattern is at28=12.6° (d=7.09 Å), while a prominent line in the zeolite L XRD patternis at 28=22.7° (d=3.91 Å). The relative peak intensities of these peakscan be compared to determine the relative proportions of the two zeolitetypes, since these peaks are not obscured in mixtures of the twozeolites. It is a preferred feature that zeolite of the invention has anXRD pattern in which the peak height ratio (d=7.09 Å)/(d=3.91 Å) is notgreater than 0.2. Very preferably the product is substantially free ofzeolite W as evidenced by an absence of the XRD pattern of a line at a dspacing of 7.09 Å. As described in EP0096479, it was previously thoughtnecessary to avoid contamination for the reaction mixture to comprisethe reactants in the following molar ratios:

    M.sub.2/n O/SiO.sub.2 =>0.25

    B.sub.2 O/M.sub.2/n O=<65

    SiO.sub.2 /Al.sub.2 O.sub.3 =7.5-10.5

It is a surprising feature of the present invention that a highlycrystalline product of the invention, substantially free of zeolite Wmay be obtained from a reaction mixture outside the ranges taught by theprior art, and specifically in which:

    M.sub.2/n O/SiO.sub.2 <0.25 and/or

    H.sub.2 O/M.sub.2/n O>65 and/or

    SiO.sub.2 /Al.sub.2 O.sub.3 =5-7.5

by selecting a reaction mixture such that M is a mixture of M'+M¹¹where:

    M.sup.1.sub.2 O/(M.sup.1.sub.2 O+M.sup.11.sub.2/n O)=0.900-0.9999

Thus, in another aspect this invention provides a process for thepreparation of zeolite L, in which a reaction mixture comprising water,a source of alkali metal, a source of silicon, a source of aluminium anda source of an additional metal M¹¹ is heated to a temperature of atleast 75° C. to form the desired zeolite L, the reaction mixture beingsuch that in the absence of the additional metal M¹¹ the formed productwould comprise substantial amounts of zeolite W, in which the presenceof the additional metal M¹¹ results in reduced contamination by zeoliteW. It is surprisingly found that very small amounts (even in somesystems as low as a few parts per million) of the additional metal areeffective at surpressing zeolite W, though the zeolite W level in theproduct depends upon other components of the reaction mixture.

The effect of the additional metal M¹¹ in supressing the tendency tozeolite W formation in a zeolite L reaction gel is extremely valuablesince it effectively expands the range of reaction mixtures which can beused to obtain a high quality zeolite L product. The effect of M¹¹ insuppressing zeolite W formation may also be demonstrated by usingdifferent crystallisation conditions. In general, a zeolite Lcrystallisation gel is extremely susceptible to zeolite W contaminationif subjected to shear forces--for example, by stirring the gel duringcrystallisation. We have now found that zeolite L may be prepared in astirred crystallisation by incorporating additional metal M¹¹ in thegel. The invention extends to a stirred process for zeolite L formationin which the tendency to zeolite W formation is be prevented byincorporation of a metal M¹¹ in the reaction mixture. The reactionmixture may, for example, be a gel as described in EP0096479 to which azeolite W-suppressing amount of M¹¹ is added.

Thus, in a further aspect, this invention provides a method ofsupressing zeolite W formation in the preparation of zeolite L from acrystallisation gel in which the gel composition and/or crystallisationconditions such as stirring would otherwwise allow zeolite W formation,which method comprises introducing into the gel a zeolite W-suppressingamount of a source of an additional metal M¹¹. The zeolite W-suppressingamount is as indicated above surprisingly small, and since theadditional metal also tends to result in smaller crystallite size in theproduct, it will not be desirable to increase the amount of additionalmetal beyond the level at which zeolite W is suppressed if smallercrystallites are not wanted. It has been found that the best results areobtained at extremely low, but non-zero, amounts of additional metal.

The alkali metal M' is very preferably potassium (K), but may be amixture of potassium with other alkali metals, for example sodium. It isa further surprising feature of the invention that a greater degree ofreplacement of potassium by other alkali metals in possible in thepresence of the additional metal without significant amounts of zeoliteW being formed in the Zeolite L product. EP0096479 indicates that thepreferred maximum amount of alkali metal other than potassium is 30 mole% of the total alkali metal content. We have found that at this level ofother alkali metal and even at greater levels the tendency to formzeolite W may be substantially completely surpressed by the presence ofthe additional metal M¹¹.

Thus, the preferred zeolites of the invention may be obtained within thefollowing preferred ranges:

    ______________________________________                                        (M.sup.1.sub.2 O + M.sup.11.sub.2/n O)/SiO.sub.2                                                 0.18-0.26                                                  H.sub.2 O/(M.sup.1.sub.2 O + M.sup.11.sub.2/n O)                                                 50-90                                                      SiO.sub.2 /Al.sub.2 O.sub.3                                                                       6-12                                                      M.sup.1.sub.2 O/(M.sup.1.sub.2 O + M.sup.11.sub.2/n O)                                            0.959-0.9999                                              ______________________________________                                    

where M¹ is potassium or a mixture of potassium and a second alkalimetal M² and K₂ O/K₂ O+M₂ O=0.5-1.

The amount of the additional metal M¹¹ may be very low, only a few ppmof the reaction mixture, and yet still have an effect in promotingzeolite L formation, forming smaller and/or more cylindrical zeolite Lparticles and/or promoting the properties of the zeolite L product. Thusin a further aspect this invention provides a process of preparingzeolite L in which there is added to the synthesis gel prior tocrystallization a source of an additional metal M¹¹, in an amount suchthat the amount of additional metal in the gel is from 0.1 ppm to 0.1 wt% of the gel, preferably from 5 ppm to 0.05 wt % of the gel.

In addition to varying the proportions of the reactants in the reactionmixture, it is possible to vary the reaction conditions and inparticular the crystallisation temperature. By using differenttemperatures, it may be possible to deviate further from the preferredcomposition defined above and yet still obtain the desired product. Ingeneral, within the broad reactant ratios defined for the process of theinvention, a higher crystallisation temperature enables the siliconcontent to be lowered and/or the water content to be lowered and/or thepotassium content (and thus the alkalinity) to be raised. By contrast,operating at lower temperatures tends to decrease the nucleation ratewhich can be countered by lowering the alkalinity and/or by increasingthe water content and/or by introducing seeds of preformed zeolite L.Increasing the alumina content enables higher yields of zeolite L to beobtained, whereas the prior art teaches that this leads to zeolite Wformation.

The additional metal M¹¹ may be introduced as any convenient compoundsuch as an oxide, carbonate, silicate hydroxide or sulphate. Bariumsulphate has been found an effective source of barium even though notsoluble in the reaction medium. The additional metal M¹¹ is preferablymagnesium, calcium, barium, zinc or cobalt though chromium manganese ornickel may also be employed.

The function of the additional metal is not wholly understood but atleast some of the metals are believed to form highly insoluble silicateswhich may form very fine suspensions of silicate particles which willfunction as nuclei or seeds for zeolite L formation. Thus additionalmetals forming highly insoluble silicates are preferred.

In the synthesis of all zeolitic materials of the invention, the sourceof silicon for the reaction mixture is generally silica, and this isusually most conveniently in the form of a colloidal suspension ofsilica such as Ludox HS 40 available from E. I. Dupont de Nemours andCo. Colloidal silica sols are preferred since they result in lesscontaminating phases. However, other forms such as silicates may beused.

The source of aluminium may be an alumina introduced into the reactionmedium as, for example, Al₂ O₃.3H₂ O, previously dissolved in alkali.However, it is also possible to introduce aluminium in the form of themetal, which is dissolved in alkali.

The aluminosilicates of the invention are preferably obtained fromreaction mixtures containing potassium. This potassium is preferablyintroduced as potassium hydroxide.

The product of the processes described above is a mixed cation form ofthe zeolite containing alkali metal, preferably potassium, and metalM¹¹. The molar ratio of K₂ O/(K₂ O+M¹¹ _(2/n) O) in the product of theinvention is preferably greater than 0.95, more preferably greater than0.98. The amount of the cation M¹¹ in the zeolite is preferably lessthan 0.1 wt % of the zeolite L, and may be below 0.05 wt % of thezeolite L. By ion exchange of the product in the manner conventional tozeolite chemistry, other cations can be introduced. However it is asurprising feature of the invention that the cation M¹¹ cannot becompletely replaced by ion exchange indicating that some of them atleast of the M¹¹ cations are in non-exchangeable sites in the zeolite Lstructure.

Within the ranges specified hereinbefore for the composition of thereaction mixture, it is possible to choose ratios of oxides andalkalinity to given particular forms of the aluminosilicate product. TheSiO₂ /Al₂ O₃ ratio in the reaction mixture may vary over a wide rangebut the SiO₂ /Al₂ O₃ ratio in the product preferably lies in arelatively narrow range of 5.4 to 7.4. The higher the SiO₂ /Al₂ O₃ ratioin the reaction mixture, the higher the ratio in the product. Also,decreasing alkalinity (OH⁻ /SiO₂) tends to increase the SiO₂ /Al₂ O₃ratio in the formed product. Dilution of the reaction mixture with waterand thus increasing the H₂ O/K₂ O ratio also tends to increase the SiO₂/Al₂ O₃ ratio in the product.

Particle size is also affected by the composition of the reactionmixture and the nature of the raw materials used. Generally, theparticles formed are in the range of from 0.05 to 4.0μ, but the use ofthe metal M¹¹ tends to favour small particles, despite the lowalkalinity of the synthesis gel. However, even in the presence of M¹¹larger, but still small, particle sizes are favoured by loweralkalinity. Higher dilution and high temperatures tend to favourformation of particles with an increased m/d ratio.

Crystallisation time is related to the crystallisation temperature. Thecrystallisation is preferably carried out in the region of 150° C. andat this temperature the crystallisation time may be from 24 to 96 hours,typically from 48 to 72 hours. Lower temperatures may require muchlonger times to achieve good yield of the desired product, whereas timesof less than 24 hours are possible when higher temperatures are used. Atime of 8 to 15 hours is typical for a temperature of 200° C. orgreater.

The crystallisation is generally carried out in a sealed autoclave andthus at autogenous pressure. It is generally inconvenient, althoughpossible, to employ higher pressures. Lower pressure will require longercrystallisation times.

Following the preparation as described above the zeolite L may beseparated, washed and dried in the normal manner.

The product of the processes of the invention described hereinbefore arepreferably substantially free from contaminant crystalline and amorphousmaterials. However, in employing these products in catalyticapplications it may be desired to combine them with additionalcrystalline or amorphous materials and this invention extends to suchcombinations.

We have found that the zeolite L of the invention is an excellentcatalyst base and may be used in a wide variety of catalytic reactions.The particular morphology of the crystals appears to result in aparticular stable base for catalytically active metals with a surprisingresistance to metal catalyst deactivation. In addition, the zeolite L ofthe invention has displayed low acidity which makes it especially suitedto catalytic applications where a low acid site strength is advantageoussuch as aromatisation.

The catalytically-active metal(s) may be, for example, a Group VIIImetal such as platinum, tin, or germanium as described in U.S. Pat. No.4,104,320, or a combination of platinum and rhenium as described in GB2,004,764 or BE 888365. In the latter case, the catalyst may forappropriate circumstances also incorporate halogen as described in U.S.Pat. No. 4,165,276, silver as described in U.S. Pat. Nos. 4,295,959 and4,206,040, cadmium as described in U.S. Pat. Nos. 4,295,960 and4,231,897 or sulphur as described in GB 1,600,927.

We have found a particularly advantageous catalyst composition toincorporate from 0.1 to 6.0 weight %, preferably from 0.1 to 1.5 weight% platinum or palladium, since this gives excellent results inaromatisation. From 0.4 to 1.2 weight % platinum is particularlypreferred, especially in conjunction with the potassium form of thealuminosilicate. The invention extends to catalysts comprising thezeolitic material and a catalytically-active metal.

It may also be useful to incorporate into the catalyst of the inventionone or more materials substantially inert under the conditions in whichthe catalyst is to be employed to act as a binder. Such binders may alsoact to improve the resistance of the catalyst to temperature, pressureand attrition.

The zeolite L of the invention may be used in a process for theconversion of a hydrocarbon feed in which the feed is contacted with acatalyst as described above under appropriate conditions to bring aboutthe desired conversion. They may, for example, be useful in reactionsinvolving aromatisation and/or dehydrocyclisation and/or isomerisationand/or dehydrogenation reaction. They are particularly useful in aprocess for the dehydrocyclisation and/or isomerisation of aliphatichydrocarbons in which the hydrocarbons are contacted at a temperature offrom 370° to 600° C., preferably 430° to 550° C., with a catalystcomprising zeolite L of the invention, preferably having at least 90% ofthe exchangeable cations M as alkali metal ions, and incorporating atleast one Group VIII metal having dehydrogenating activity, so as toconvert at least part of the aliphatic hydrocarbons into aromatichydrocarbons.

The aliphatic hydrocarbons may be straight or branched chain acyclichydrocarbons, and particularly paraffins such as hexane, althoughmixtures of hydrocarbons may also be used such as paraffin fractionscontaining a range of alkanes possibly with minor amounts of otherhydrocarbons. Cycloaliphatic hydrocarbons such as methylcyclopentene mayalso be used. In a preferred aspect the feed to a process for preparingaromatic hydrocarbons and particularly benzene comprises hexanes. Thetemperature of the catalytic reaction may be from 370° to 600° C.,preferably 430° to 550° C. and preferably pressures in excess ofatmospheric are used, for example up to 2000 KPa, more preferably 500 to1000 KPa. Hydrogen is employed in the formation of aromatic hydrocarbonspreferably with a hydrogen to feed ratio of less than 10.

The process is preferably otherwise carried out in the manner describedin U.S. Pat. No. 4,104,320, BE 888365, EP 40119, EP 0142351, EP 0145289or EP0142352.

It has been found that use of the zeolite L of the invention in this wayenables greatly improved catalyst lifetimes to be achieved as comparedto the lifetime obtained with a conventionally prepared zeolite.

The invention will now be described in more detail, though only by wayof illustration, in the following examples and evaluations.

COMPARATIVE EXAMPLE 1 Preparation of Zeolite L

Zeolite L was prepared according to the procedure of EP 0096479. Asynthesis gel was prepared having the following composition expressed inmoles of pure oxide:

    2.60K.sub.2 O:Al.sub.2 O.sub.3 :10SiO.sub.2 :160H.sub.2 O

This gel was prepared as follows:

The aluminium hydroxide was dissolved by boiling in an aqueous solutionof potassium hydroxide pellets (86% pure KOH) to form Solution A. Afterdissolution any water loss was corrected. A separate solution, SolutionB, was prepared by diluting colloidal silica (Ludox HS 40) with water.

Solutions A and B were mixed for two minutes to form a gel, and justbefore the gel became fully stiff, it was transferred to a Teflon-linedautoclave, preheated to 150° C. and held at that temperature for 72hours to bring about crystallisation.

The formed zeolite L was highly crystalline with a typical zeolite Lx-ray diffraction (XRD) pattern. Scanning electron micrographs (SEM)show the product to be formed solely of cylindrical crystals having amean diameter of 1 to 2 microns, an aspect ratio (m/d) of 0.5-1 and anm/h ratio of 0.65-0.85. The SiO₂ :Al₂ O₃ ratio in the product was 6.3.

COMPARATIVE EXAMPLE 2-6 Variation of potassium content

As shown in Table 1, the variation of the amount of potassium in thecase where M=K was also investigated. Variation of potassium contentfrom 2.41 moles K₂ O (Example 3) to 2.75 moles K₂ O (Example 2) gavezeolite L with a cylindrical form, but with m/h<1. Example 4 gavezeolite L with a morphology intermediate a clam shape and the cylindershape--that is, m/h was very much less than 1.

A low potassium content of 2.15 moles K₂ O (Example 5) gave a productwith low crystallinity. A high potassium content of 3.4 moles K₂ O(Example 6) gave a clam-shaped product.

EXAMPLE 7 Preparation of zeolite L of the invention

The procedure of Example 1 was modified by the addition of certain metalcations to the synthesis gel.

Solution A was prepared from:

    ______________________________________                                        KOH (87.5% KOH) pellets                                                                            30.81 g                                                  Aluminium hydroxide  15.61 g                                                  Water                49.82 g                                                  Additional water     25.39 g                                                  ______________________________________                                    

The aluminium hydroxide powder was dissolved in the potassium hydroxidesolution by boiling. After cooling to ambient temperature, the waterloss was corrected.

Solution B was prepared from:

    ______________________________________                                        Barium hydroxide Ba(OH).sub.2.8H.sub.2 O                                                              0.17 g                                                Silica (Ludox HS 40)   150.16 g                                               Water                  120.32 g                                               Seeds of zeolite L from Example 1                                                                     0.75 g                                                ______________________________________                                    

Barium hydroxide powder was added to the colloidal silica solution,which gelled on mixing. The synthesis gel was seeded with pre-formedzeolite L crystallites to enhance the crystallization process, but suchseeding is not essential.

Solution A was added to Solution B and mixed for 5 minutes to form thesynthesis gel. This had the composition (in moles of oxides):

    2.4K.sub.2 O:0.005BaO:Al.sub.2 O.sub.3 :10SiO.sub.2 :164H.sub.2 O

Crystallisation was conducted at 175° C. for 73 hours. The product waswashed 4 times with 500 cm³ water. The pH of the water from the lastwashing was 10.6. The product was dried at 125° C. and the results aregiven in Table 3. The barium content of the product was measured to beabout 300 ppm by atomic adsorption spectroscopy, and about 80 ppm by ionplasma emmission spectroscopy.

EXAMPLES 8-17 Preparation of zeolite L of the invention

The procedure of Example 7 was repeated with different amounts of bariumpresent and using magnesium and calcium in place of barium, with andwithout seeding. The synthesis conditions are set out in Table 2 and thedetails of the products are given in Table 3.

                                      TABLE 1                                     __________________________________________________________________________           Gel Composition                                                                            Product                                                   Comparative                                                                          (moles)      (wt %)                                                    Example                                                                              K.sub.2 O                                                                        Al.sub.2 O.sub.3                                                                  SiO.sub.2                                                                        H.sub.2 O                                                                        K.sub.2 O                                                                        Al.sub.2 O.sub.3                                                                  SiO.sub.2                                                                        H.sub.2 O                                       __________________________________________________________________________    2      2.75                                                                             1   10 160                                                                              6.7                                                                              2.7 15.6                                                                             75.0                                                                             cylindrical zeolite L (m/h < 1)              3      2.41                                                                             1   10 160                                                                              5.9                                                                              2.7 15.8                                                                             75.6                                                                             cylindrical zeolite L (m/h < 1)              4      3.01                                                                             1   10 160                                                                              7.3                                                                              2.6 15.5                                                                             74.5                                                                             clam/cylinder zeolite L (m/h << 1)           5      2.15                                                                             1   10 160                                                                              5.3                                                                              2.7 15.9                                                                             76.1                                                                             low crystallinity product                    6      3.4                                                                              1   10 160                                                                              8.2                                                                              2.6 15.4                                                                             73.8                                                                             clam-shaped zeolite L                        __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________    Synthesis Gel                                                                 (moles)                   Crystallization Conditions                          Example                                                                            M.sup.11                                                                         K.sub.2/n O                                                                       M.sup.11.sub.2 O                                                                  Al.sub.2 O.sub.3                                                                  SiO.sub.2                                                                        H.sub.2 O                                                                        Temp (°C.)                                                                   Time (hrs)                                                                            Gel Seeded                            __________________________________________________________________________     7   Ba 2.40                                                                              0.005                                                                             1   10 164                                                                              175   73      yes                                    8   Ba 2.60                                                                              0.01                                                                              1   10 159                                                                              175     87.5  yes                                    9   Ba 2.60                                                                              0.05                                                                              1   10 160                                                                              150 + 175                                                                           73 + 49 no                                    10   Ba 2.60                                                                              0.2 1   10 160                                                                              150   73      yes                                                             (+175 +24 no change)                                11   Mg 2.29                                                                              0.1 1   10 161                                                                              150   68      yes                                   12   Mg 2.59                                                                              0.1 1   10 163                                                                              150   65      yes                                   13   Ca 2.58                                                                              0.1 1   10 162                                                                              150   65      yes                                   14   Ca 2.29                                                                              0.1 1   10 162                                                                              150   135     yes                                   15   Ba 2.21                                                                              0.005                                                                             1   10 162                                                                              175   65      no                                    16   Ba 2.10                                                                              0.005                                                                             1   10 161                                                                              175   77      no                                    17   Ba 2.01                                                                              0.005                                                                             1   10 162                                                                              175   77      no                                    __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________                        Crystallinity                                             Product             (relative to               Aspect                         (moles)             Ex. 1) W/L a)                                                                             Zeolite    Mean                                                                              Ratio                          Example                                                                            K.sub.2 O                                                                        M.sup.11.sub.2/n O                                                                 Al.sub.2 O.sub.3                                                                  SiO.sub.2                                                                        %      ratio                                                                              Type                                                                              Morphology                                                                           Dia(μ)                                                                         L/d L/h                        __________________________________________________________________________     7   1.04                                                                              0.0001                                                                            1   6.7                                                                              99     <0.01                                                                              L   cylinder                                                                             0.3-0.5                                                                           1-3 1                           8   1.00                                                                             0.005                                                                              1   6.4                                                                              81     <0.01                                                                              L   cylinder                                                                             0.2                                 9   not analyzed   50     <0.01                                                                              L          0.05-0.1                                                                          ?   ?                          10   not analyzed   40     <0.01                                                                              L   cylinder                                                                             0.1-0.3                            11   1.07                                                                             0.04 1   6.9                                                                              92     <0.01                                                                              L   cylinder                                                                             0.3-0.5                                                                             1-1.5                                                                           0.85-1                     12   1.11                                                                             0.04 1   6.8                                                                              84     <0.01                                                                              L   cylinder                                                                             0.2-0.3                                                                           0.8-1                                                                             ?                          13   1.08                                                                             0.05 1   7.0                                                                              62     <0.01                                                                              L   cylinder                                                                             0.2-0.4                            14   alien phase     0     <0.01                                                                              no L                                                                              plate  --  --  --                         15   0.90                                                                             0.004                                                                              1   6.3       <0.01                                                                              L   cylinder                                                                             0.2-0.4                                                                           1-4 1                          16   0.98                                                                             0.003                                                                              1   6.7                                                                              76      0.45                                                                              L   cylinder                                                                             0.3-0.5                                                                           1-4 1                          17   1.12                                                                             0.004                                                                              1   8          0.8 L   cylinder                                                                             0.3-0.6                                                                           2-6 1                          __________________________________________________________________________     *"?" indicates that the parameter could not be determined at the              resolution of the microscopic analysis used.                                  a) ratio XRD peak intensity of zeolite W at d = 7.09Å/zeolite L at d      22.7                                                                     

The products obtained are in Examples 8 to 13--small cylindricalparticles of zeolite L. In Example 14, no zeolite L was seen after 65hours, and by extending the crystallisation time, an alien phase(unidentified) was produced. Thus, for calcium addition, the gel andcrystallisation conditions were inappropriate for zeolite L formation,but Example 13 shows that for that particular additional metal at higherpotassium levels, zeolite L is produced. It is necessary to select thecrystallisation temperature and time to optimise zeolite L preparationfor a given synthesis gel composition, and shorter crystallisation timeswith the Example 14 gel may increase zeolite L production.

EXAMPLE 18 Stirred zeolite L synthesis

The procedure of Example 8 was repeated except that the crystallisationwas carried out for 136 hours at only 100° C. while stirring at 25 rpm.By way of comparison, a synthesis was conducted under the sameconditions using gel containing no barium but otherwise identical.

    ______________________________________                                               BaO/Al.sub.2 O.sub.3                                                                  Crystallization                                                                             Zeolite d(μ)                                  ______________________________________                                        Example 18                                                                             0.01      100° C./136 hours/                                                                   L     0.1-0.2                                                   25 rpm                                                     Comparison                                                                             0         100° C./144 hours;                                                                   W                                                               25 rpm                                                     ______________________________________                                    

Conducting the synthesis of Example 18 without barium and withoutstirring at 100° C. takes in excess of 400 hours to produce zeolite L.

EXAMPLES 19 to 21 Effect of additional metal on particle size

The procedure of Example 7 was repeated using synthesis 5 gels with amolar composition of:

    2.39K.sub.2 O/xBaO/Al.sub.2 O.sub.3 /10SiO.sub.2 /164H.sub.2 O

in which the Ba content x was varied from 0.01 to 0.04. These werecrystallized for 72 hours at 170° C. in 300 ml stainless steelautoclaves. The resulting crystalline products were examined by X-raydiffraction and scanning electron microscopy. The products were pureZeolite L. As shown in the table below, the crystallite size wasgradually reduced with increasing Ba content in the gel:

    ______________________________________                                        Influence Ba content in gel on KL's                                           crystallite size                                                                                        Crystallite size                                               Ba content     length * width                                      Example    (moles/10 moles SiO.sub.2)                                                                   (microns)                                           ______________________________________                                         7          0.005         0.7 * 0.4                                           19         0.01           0.4 * 0.2                                           20         0.02           0.2 * 0.2                                           21         0.04           0.1 * 0.1                                           ______________________________________                                    

EXAMPLE 22 and Comparative Example 23 Effect of Alumina Content

The procedure of Example 7 was repeated using synthesis gels with thefollowing compositions:

    ______________________________________                                        Ex 22:   2.41K.sub.2 O: 0.005BaO: 1.51Al.sub.2 O.sub.3: 10SiO.sub.2 :                  162H.sub.2 O                                                         Comp Ex 23                                                                             2.41K.sub.2 O: 0.005BaO: 1.76Al.sub.2 O.sub.3 : 10SiO.sub.2 :                 163H.sub.2 O                                                         ______________________________________                                    

without the addition of seeds. These gels were crystallized for 57 hoursat 175° C.

Example 22, which the prior art suggests would result in zeolite Wformation because of the high alumina content, gave a 21.4 wt % yield ofzeolite L. XRD investigations showed this product to be highlycrystalline zeolite L with a low level of zeolite L contamination. W/Lratio (as defined hereinbefore) is measured as 0.09.

Comparative Example 23 gave a pure zeolite W product, indicating that aSiO₂ /Al₂ O₃ of 5.68 was too low for the alkalinity and water contentchosen.

EXAMPLES 24 AND 25 Zeolite L synthesis in the presence of zinc EXAMPLE24

A synthesis gel with a molar composition of:

    2.31K.sub.2 O/0.1ZnO/Al.sub.2 O.sub.3 /10SiO.sub.2 /160H.sub.2 O

containing 0.1 wt % preformed KL crystallites as a seed was crystallizedat 150° C. for 65 hours in a 300 ml stainless steel autoclave.

XRD showed that the product was an excellent crystalline (82% vsComparative Example 1) product free of zeolite W and other crystallinecontaminants. SEM showed that the product consisted of 0.2 microncylindrical crystallites with an m/d ratio between 1 and 2.

EXAMPLE 25

In this experiment the Zn content in the gel was, compared with example1, reduced by 80%, i.e. the gel composition was:

    2.31 K.sub.2 /0.02 ZnO/Al.sub.2 O.sub.3 /10 SiO.sub.2 /161 H.sub.2 O

This gel was aged at 150° C. for 67.5 hours in a 300 ml stainless steelautoclave. XRD showed that the product again was pure zeolite L withoutcontaminants. The XRD crystallinity of the product was 103% vscomparative Example 1 indicating the presence of larger crystallites.SEM indeed showed that the product consisted of 0.7 micron cylindricalcrystallites with an m/d ratio of 1.5. The cylindrical crystallites hada `perfect` appearance--i.e. very flat basal planes.

This demonstrates that

even trace amounts of Zn²⁺ species sufficiently supress the formation ofzeolite-W and

the crystallite size can be finely tuned by adjusting the Zn content inthe synthesis mixture.

EXAMPLE 26 Unseeded synthesis

The procedure of Example 7 was repeated but without the addition ofseeds. A substantially similar zeolite L product was obtained with thecharacteristics and properties set out in Example 7.

EXAMPLE 27 Use of Different Barium Salts

Example 26 was repeated replacing barium hydroxide by a series ofinsoluble barium salts as the source of barium. The synthesis gel andcrystallization conditions were otherwise identical to those used inExample 26. The results are given below:

    ______________________________________                                                                   W/L                                                Ba Source  Product         Ratio   L/d  L/n                                   ______________________________________                                        Barium Sulphate                                                                          Cylindrical zeolite L                                                                         <0.01   >1   1                                     Barium Carbonate                                                                         Cylindrical zeolite L                                                                         <0.01   >1   1                                     Barium Silicate                                                                          Cylindrical zeolite L                                                                         <0.01   >1   1                                     ______________________________________                                    

Despite these barium sources being insoluble in water the synthesis isin each case effective in producing the cylindrical crystallites ofzeolite L having flat basal planes.

EXAMPLES 28-30 AND COMPARATIVE EXAMPLES 31 AND 32 Replacement ofPotassium by Sodium

The procedure of comparative Example 1 was repeated replacing 30% and40% (molar) of the potassium by sodium in the synthesis gel, byemploying sodium hydroxide in place of the appropriate proportion ofpotassium hydroxide. In each case two syntheses were carried out, onewith the addition of barium hydroxide (in an amount of 0.005 moles BaOper mole of Al₂ O₃) and one without. The synthesis gels were eachcrystallized for 67.5 hours at 150° C. in a stainless steel autoclave.The results are given below:

    ______________________________________                                                             Ba                                                              Na.sub.2 O/K.sub.2 O + Na.sub.2 O                                                           (moles BaO/       W/L                                    Example                                                                              (molar ratio) mole Al.sub.2 O.sub.3)                                                                   Product                                                                              Ratio                                  ______________________________________                                        28     0.3           0.005      Zeolite L                                                                             0.02                                                                  99%                                                                           crystal-                                                                      line*                                         29     0.4           0.005      Zeolite L                                                                             0.02                                                                  99%                                                                           crystal-                                                                      line*                                         30     0.5           0.005      Zeolite L                                                                            0.3                                    Comp. 31                                                                             0.3           0          Zeolite L                                                                            0.3                                    Comp. 32                                                                             0.4           0           ?     0.5                                    ______________________________________                                         *Crystallinity relative to Comp. Example 1.                              

The Examples show that in the presence of large amounts of sodium whichotherwise tend to lead to zeolite W formation, the introduction of smallamounts of barium enhances zeolite L formation.

EXAMPLES 33-36 AND COMPARATIVE EXAMPLES 37-41 Investigation of DifferntAdditional Metals

Synthesis gels were prepared by the general procedure of Example 1 withthe following gel composition, expressed in terms of moles of oxides:

    2.35K.sub.2 O: Al.sub.2 O.sub.3 : 10 SiO.sub.2 : 160 H.sub.2 O

to which had been added small amounts of various additional metals asset out below. The gels were crystallized for 66 hours at 175° in a 300ml stainless steel autoclave. The product obtained is also indicated inTable A below.

These results show that the additional metals of the process of theinvention give reduced zeolite W formation and smaller crystals ofzeolite L than can be obtained in the absence of additional metal or inthe presence of Hg, Cd, Fe or La.

EXAMPLES 42-44 Variation of Cobalt Concentration

The procedure of Example 34 was repeated with reduced amounts of addedcobalt, introduced as cobalt nitrate hexahydrate. The results obtainedare set out in Table B below.

This shows that the size reduction obtained with cobalt (of ComparativeExample 41) is even greater than obtained with barium.

EXAMPLES 45-52 Variation in Magnesium Concentration

The procedure of Example 34 was repeated but replacing cobalt withvarious concentrations of magnesium, added either as magnesium hydroxideor magnesium nitrate. The results obtained are set out in Table C below.

                                      TABLE A                                     __________________________________________________________________________                        Product     Cylinder                                      Additional metal    % Crystallinity                                                                       W/L      L   d                                    Example                                                                            Source  moles/10SiO.sub.2                                                                    (vs. Ex. 1)                                                                           Ratio                                                                             Zeolite L                                                                          (μ)                                                                            (μ)                               __________________________________________________________________________    33   MnCl.sub.2.4H.sub.2 O                                                                 0.051  84      0.01                                                                              Yes  0.4-0.7                                                                            0.2-0.5                             34   Co(No.sub.3).sub.2.6H.sub.2 O                                                         0.050   43*    0   Yes  0.1 0.05                                 35   Ni(No.sub.3).sub.2.6H.sup.2 O                                                         0.025  82      0   Yes  0.2-0.6                                                                            0.2-0.4                             36   Cr(NO.sub.3).sub.3.9H.sub.2 O                                                         0.025  98      0.02                                                                              Yes  3-4 1.5-2                                Comp. 37                                                                           HgCl.sub.2                                                                            0.050  76      0.23                                                                              Yes  3-4 1.5-2                                Comp. 38                                                                           CdSO.sub.4.8H.sub.2 O                                                                 0.050  81      0.49                                                                              Yes  3-4 1.5-2                                Comp. 39                                                                           FeSO.sub.4.7H.sub.2 O                                                                 0.027  93      0.10                                                                              Yes  3-4 1.5-2                                Comp. 40                                                                           La(OH).sub.3                                                                          0.026  82      0.15                                                                              Yes  3-4 1.5-2                                Comp. 41                                                                            --     0      98      0.10                                                                              Yes  3-4 1.5-2                                __________________________________________________________________________     *Apparent low crystallinity as measured by peak height, is caused by peak     height reduction resulting from very small crystals rather than amorphous     product.                                                                 

                                      TABLE B                                     __________________________________________________________________________    Co consn.         Product                                                     Co/10SiO.sub.2    % crystallinity                                                                             Cylinders                                     Example                                                                            (moles)                                                                             in gel (ppm)                                                                         (vs. Ex. 1)*                                                                          W/L Ratio                                                                           L(μ)                                                                           d(μ)                                                                           L/h                                   __________________________________________________________________________    42   0.0051                                                                              75     70      0     ≦0.2                                                                       ≦0.15                                                                      ˜1                              43   0.0013                                                                              20     75      0     0.2-0.3                                                                           0.1-0.2                                                                           ˜1                              44   0.0002                                                                               3     90      0     0.5-0.8                                                                           0.3-0.5                                                                           ˜1                              __________________________________________________________________________     *Product crystallinity measured by peak height is apparently lower as a       result of small crystals giving reduced peak height. High resolution          microscopy indicated no significant amount of amorphous material.        

                                      TABLE C                                     __________________________________________________________________________               Mg consn.                                                          Mg         Mg/10SiO.sub.2                                                                             Product                                                                             Cylinders                                       Example                                                                            source                                                                              (moles)                                                                             in gel ppm                                                                           W/L ratio                                                                           L(μ)                                                                           d(μ)                                                                           L/h                                     __________________________________________________________________________    45   Mg(OH).sub.2                                                                        0.050 315    0     0.5-1                                                                             0.3-0.5                                                                           ˜1                                46   Mg(NO.sub.3).sub.2                                                                  0.050 315    0     ≦0.2                                                                       ≦0.1                                                                       ˜1                                47   Mg(NO.sub.3).sub.2                                                                  0.0051                                                                               35    0     0.3-0.6                                                                           0.2-0.3                                                                           ˜1                                48   Mg(NO.sub.3).sub.2                                                                  0.0026                                                                               20    0     0.4-0.7                                                                           0.3-0.4                                                                           ˜1                                49   Mg(NO.sub.3).sub.2                                                                         9     0     0.5 0.1-0.3                                                                           ˜1                                50   Mg(NO.sub.3).sub.2                                                                         6     0     0.7 0.2-0.4                                                                           ˜1                                51   Mg(NO.sub.3).sub.2                                                                         3     0     1   0.5-0.6                                                                           ˜1                                __________________________________________________________________________

These results show the effect of even small amounts of magnesium atpromoting the formation of zeolite L, since a similar synthesis withoutadded magnesium (Comparative Example 41) resulted in significant amountsof zeolite W. The results also show the effect of increasing amounts ofmagnesium in decreasing the size of the formed zeolite L crystallites.

EXAMPLE 52 Effect of magnesium on an aluminium-rich synthesis

The procedure of Comparative Example 1 was repeated to prepare asynthesis gel of the composition

    2.50K.sub.2 O:1.60AL.sub.2 O.sub.3 :10SiO.sub.2 :10SiO.sub.2 :165H.sub.2 O

to which had been added 15 ppm Mg, as Mg(NO₃)₂ according to theprocedure of Example 11. The gel was crystallized in an autoclave at125° C. for 23 hours and at 175° C. for a further 66 hours.

The process yielded more than 20% of a zeolite L product with a W/Lratio of 0, thus with no contaminating zeolite W which EP0096479 hasshown to be formed at high levels of Al₂ O₃ in the absence of additionalmetal. The product was in the form of cylindrical crystallites with alength of 0.3-0.5 microns and a diameter of 0.2-0.3 microns.

Evaluation: Aromatisation

The performance of certain of the zeolite L samples of the examples as acatalyst base in aromatisation was compared to products of EP0096479. Ineach case, a catalyst was prepared by impregnating the base with 0.64weight % platinum by an incipient wetness technique in which the basewas dried at 140° C. overnight then added to a 1.68 wt % solution ofplatinum tetraamine dichloride monohydrate, stirred and aged at roomtemperature for 30 minutes, then dried in a vacuum oven at 120° C. for 4hours. The impregnated sample was dried at 110° C. for 4 hours, and thenpelletised, crushed to 16-45 mesh (US seive size) and loaded into avertical tubular reactor, calcined in air (substantially water-free) at480° C., and then reduced in hydrogen at 510° C.

An aromatisation screening test was carried out at a temperature of 510°C. and 738 KPa (107 psig) pressure with a C₆ mixed feed comprising:

    ______________________________________                                        Component        Weight %                                                     ______________________________________                                        iso-C.sub.6      60                                                           (3-methyl-pentane)                                                            n - C.sub.6      40                                                           ______________________________________                                    

at a weight hourly space velocity of 8.0 w/w hr⁻¹ and in the presence ofhydrogen, the H₂ :hydrocarbon ratio being 4. This is an accelerated testwhich provides a good correlation with catalyst performance incommercial aromatisation. The results are given in Table 4.

A catalyst using the zeolite of Example 7 as its catalyst base iscompared with a catalyst using a zeolite L prepared according toEP0096479 with cylindrical particles (d=0.7-1.4μ, m/d=0.5-1, m/h=0.67),an SiO₂ /Al₂ O₃ ratio of 5.8 and containing no additional metal M¹¹,hereinafter referred to as "Comparison"

                  TABLE 4                                                         ______________________________________                                                      Example 7 Comparison                                                          Time On Stream (hr)                                                           22     50     22       50                                       ______________________________________                                        C.sub.6 Conversion (%)                                                                        81.6     77.2   74.9   68.4                                   Benzene Yield (%)                                                                             60.2     57.7   51.5   46.6                                   Benzene Selectivity (%)                                                                       73.8     74.8   68.7   68.1                                   ______________________________________                                    

A 16% (relative) higher benzene yield was observed with the inventionwhereas no changes in C₄ -cracking pattern were observed. The higheraromatics yield is to improved activity (conversion) and to improvedselectivity.

Further aromatisation screening test were carried out on catalystsprepared using the following materials as substrate.

(a) zeolite L of Example 26;

(b) zeolite L of Example 26, exchanged with an aqueous 1N potassiumnitrate solution for 24 hours at ambient temperature and then dried at120° C. for four hours prior to impregnation with platinum as describedabove-the Ba content is reduced to 50 ppm (as measured by ion plasmaemission spectroscopy);

(c) Comparison, as defined above;

(d) Comparison, exchanged with barium as described in Example 1 of GB2116450 to give a Ba content of 100 ppm as measured by ion plasmaemmission spectroscopy, prior to impregnation with platinum as describedabove.

                  TABLE 5                                                         ______________________________________                                                        Example 7         Comparison                                          Example 7                                                                     K       Com-      Ba                                                          Scale-up                                                                              exchanged parison exchanged                                           Time on stream (hr)                                                           23      23        23      23                                          ______________________________________                                        C.sub.6 Conversion                                                                      78.7      76.3      74.9  70.1                                      (%)                                                                           Benzene yield                                                                           58.2      56.9      51.2  48.8                                      (%)                                                                           Benzene   74.0      74.6      68.4  69.6                                      selectivity (%)                                                               ______________________________________                                    

These results shown that the zeolite L of the invention is better interms of conversion, yield and selectivity than the zeolite L describedin EP 0096749, in this catalytic application. Exchanging the zeolite Lof the invention did not remove all barium from the sample, showing thatthe barium is not all in an exchangeable form, but by removing theexchangeable barium the advantage over the zeolite L of EP 0096479 isnot lost. Furthermore, carrying out the barium exchange procedure of GB2116450 on the zeolite L of EP 0096479 did not improve the performanceof the zeolite L in the terms of conversion or benzene yield, but rathermade it worse, and the change in benzene selectivity is relativelysmall. This comparison shows that the advantage of the zeolite L of theinvention does not lie in barium exchange as described in GB 2116450.

I claim:
 1. A process for the preparation of zeolite L crystallites inthe form of cylinders, containing reduced amounts of zeolite W, in whichan alkaline reaction mixture comprising water, a source of alkali metal,a source of silicon and a source of aluminum is heated to a temperatureof from at least 75° C. to form the zeolite L, characterized in that thereaction mixture comprises a source of an additional metal M¹¹ presentsuch that the amount of M¹¹ ranges from about 0.1 ppm to about 0.1 wt %based on said reaction mixture and has a composition falling within thefollowing molar ratios expressed as oxides:

    ______________________________________                                        (M.sup.1.sub.2 O + M.sup.11.sub.2/n O)/SiO.sub.2                                                0.18-0.36                                                   H.sub.2 O/(M.sup.1.sub.2 O + M.sup.11.sub.2/n O)                                                25-90                                                       SiO.sub.2 /Al.sub.2 O.sub.3                                                                      5-15                                                       M.sup.1.sub.2 O/(M.sup.1.sub.2 O + M.sup.11.sub.2/n O)                                           0.900-0.9999                                               ______________________________________                                    

wherein M¹ is potassium or a mixture of potassium and sodium, M¹¹ is acation of magnesium, calcium, barium, manganese, chromium, cobalt,nickel or zinc, and n is the valence of M¹¹.
 2. A process as claimed inclaim 1, in which the reaction mixture is heated to from 100° C. to 250°C.
 3. A process as claimed in claim 2, in which the reaction mixture isheated to from 120° C. to 225° C.
 4. A process for the preparation ofzeolite L, wherein a reaction mixture comprising water, a source ofalkali metal consisting of potassium, a source of silicon, and a sourceof aluminum is heated to a temperature of at least 75° C. to formzeolite L, characterized in that the reaction mixture contains anadditional metal M¹¹, and that the reaction mixture is such that in theabsence of the additional metal M¹¹ the final product would comprisesubstantial amounts of zeolite W, the amount of the additional metal M¹¹being in the range of from about 0.1 ppm to about 0.1 wt% based on theweight of said reaction mixture, wherein M¹¹ is a cation of magnesium,calcium, barium, manganese, chromium, cobalt, nickel, or zinc.
 5. Aprocess as claimed in claim 1, wherein the reaction mixture comprises:

    ______________________________________                                        (M.sup.1.sub.2 O + M.sup.11.sub.2/n O)/SiO.sub.2                                                 0.18-0.26                                                  H.sub.2 O/(M.sup.1.sub.2 O + M.sup.11.sub.2/n O)                                                 50-90                                                      SiO.sub.2 /Al.sub.2 O.sub.3                                                                       6-12                                                      M.sup.1.sub.2 O/(M.sup.1.sub.2 O + M.sup.11.sub.2/n O)                                            0.950-0.9999                                              ______________________________________                                    

where M¹ is potassium or a mixture of potassium and a second alkalimetal M² when the molar ratio K₂ O/K₂ O+M₂ ² O is from 0.5 to 1, whereinM¹¹ is a cation of magnesium, calcium, barium, manganese, chromium,cobalt, nickel, or zinc and n is the valence of M¹¹.
 6. A method ofsuppressing zeolite W formation during the preparation of zeolite Lhaving crystallites of reduced crystallite size from a crystallizationgel containing a source of alkali metal which is potassium or a mixtureof potassium and sodium, in which the crystallization conditions wouldotherwise allow zeolite W formation, which method comprises introducinginto the gel an amount of a zeolite W-suppressing source of anadditional metal M¹¹, wherein M¹¹ is a cation of magnesium, calcium,barium, manganese, chromium, cobalt, nickel, or zinc, said metal cationpresent in said gel in an amount of from about 0.1 ppm to about 0.1 wt %based on the weight of said gel.
 7. The method of claim 6 wherein theamount of additional metal M¹¹ is from 5 ppm to 0.05 wt % based on theweight of the gel.
 8. A zeolite L composition comprising cylindricalcrystallites having basal planes shaped such that the ratio of axiallength of curved cylindrical surface (m) to the overall axial length ofthe crystallite (h) is greater than 0.9, and the aspect ratio of (m) tothe mean diameter (d) is at least 0.5, wherein at least 80% of the basalplanes are microscopically flat to within about 200Å and do not exhibitspiral step growths thereon, said zeolite further characterized in thatit is crystallized from a synthesis mixture containing a source ofalkali metal which is potassium or a mixture of potassium and sodium andalso containing a source of additional metal selected from the groupconsisting of magnesium, calcium, barium, manganese, chromium, cobalt,nickel and zinc cations, said additional metal present in an amount offrom about 0.1 ppm to about 0.1 wt % based on said synthesis mixture. 9.The zeolite L composition of claim 8, wherein the cylindricalcrystallites have a mean diameter (d) of at least 0.05μ.
 10. The zeoliteL composition of claim 8, wherein the cylindrical crystallites have amean diameter (d) of at least 0.1μ.
 11. The zeolite L composition ofclaim 8, wherein the cylindrical crystallites have an aspect ratio of atleast
 1. 12. The zeolite L composition of claim 8, wherein at least 90%of said basal planes are microscopically flat.
 13. The zeolitecomposition of claim 12 wherein said ratio of (m) to (h) issubstantially unity.
 14. The method of claim 7 wherein said zeolite Lcomprises cylindrical crystallites having an aspect ratio of axiallength of curved cylinder surface (m) to mean diameter of at least 0.5and basal planes shaped such that the ratio of (m) to the overall axiallength of the crystallite (h) is greater than 0.9 and at least 80% ofsaid basal planes are microscopically flat to within about 200Å and donot exhibit spiral step growths therein.
 15. The method of claim 1wherein said cylindrical crystallites are right cylinders having a meandiameter in the range of from 0.1 to 0.5 microns.
 16. The method ofclaim 1 wherein the ratio of SiO₂ to Al₂ O₃ is in the range of from 5 to7.5.
 17. The method of claim 16 wherein the ratio of SiO₂ to Al₂ O₃ isin the range of from 6.5 to 7.5.
 18. The method of claim 1 wherein saidcylinders have an aspect ratio of (m) to the mean diameter (d) in therange of from 0.75 to
 5. 19. The method of claim 18 wherein saidcylindrical crystallites are right cylinders having a mean diameter inthe range of from 0.1 to 0.5 microns.
 20. The zeolite L composition ofclaim 8 wherein said cylindrical crystallites are right cylinders havinga mean diameter in the range of from 0.1 to 0.5 microns.
 21. The zeoliteL composition of claim 20 wherein said aspect ratio is in the range offrom 0.75 to
 5. 22. The zeolite L composition of claim 8 wherein theratio of SiO₂ to Al₂ O₃ is in the range of from 5 to 7.5.
 23. Thezeolite L composition of claim 22 wherein the ratio of SiO₂ to Al₂ O₃ isin the range of from 6.5 to 7.5.
 24. The zeolite L composition of claim8 wherein said additional metal is present in an amount of from 5 ppm to0.05 wt % based on the weight of said synthesis mixture.
 25. The zeoliteL composition of claim 8 wherein said alkali metal consists ofpotassium.
 26. The zeolite L composition of claim 8 wherein saidadditional metal cation is barium.
 27. The zeolite L composition ofclaim 8 wherein said additional metal cation is magnesium.
 28. Thezeolite L composition of claim 8 wherein said additional metal cation iszinc.